The reason why other country ban cryptocurrencies because its decentralized and they can’t control it if you can’t control the currency that coming to your state you can’t get regular or can’t deduct the correct tax to your people. The other reason is some people are using the advantage to do their illegal work, like selling drugs and smuggled stuff. Another reason is maybe they are creating their own cryptocurrency like another country, and they don’t want to use the other cryptocurrency that they don’t own.